According to one myth, god Apollo gave her the gift of foretelling the future and then tried to sleep with her. However, she rejected him and to punish her, he cursed her so that no one would ever believe her prophecies. A different version has it that Cassandra initially consented to sleeping with the god in exchange for the ability to foresee the future, only to break her promise after she received the gift.

After being cursed, she was met with disbelief by her family and by the Trojans. She foretold that Paris, her brother, would bring about a war that would destroy their city, if he went to Sparta. Her brother did not believe her, and upon his return from Sparta with Menelaus' wife, Helen, Cassandra attacked her for the pain that was about to be caused. She also foretold that Troy would fall by a clever machination of the Greeks, the Trojan Horse, in which they would hide; her fellow citizens did not listen to her words, thus causing the end of the city.

After the fall of Troy, she was taken by Agamemnon back to his home; despite Cassandra's warnings about the plots of Agamemnon's wife, Clytemnestra, he went back home where he was murdered by his wife and her lover, Aegisthus. When she died, her soul was sent to the Elysian Fields, a place in the Underworld where the worthy souls rested.